microwave bemidji 
An snipped from POWERLABS on creation of plasma in a microwave oven:
Microwaves will bounce off any conductive material. That can be anything from metal to carbon or even molten glass. Upon doing so, it will induce a current flow on the metal at their frequency (2.45GHz for microwave ovens). It is actually that current flow going through the metal that re-transmits the microwaves.
As electrons concentrate at the extremities of the conductive surface (electrostatic field theory, faraday's law), they easily reach field potentials above 16kv/cm and leave the surface, causing sparking and localized heating. They will also flow between two conductors placed in a microwave field. When the metal reaches a high enough temperature, it begins to vaporize (placing burning matter into the cooking cavity greatly increases this effect, which is why burning candles, toothpicks, and even cigarettes work so well), and this metal vapor will then absorb the microwaves and it too will experience a current flow, which will effectively ionize it and make it glow.
Instead of dissipating, however, this mass of ionized metal vapor will suffer the effects of the current flow that is ionizing it in such a way that it will pull itself together. It also glows because it is very hot (due to the current flow), and its color changes are attributed to its temperature and to the ions present inside it. When the microwave energy being inputted equals the heat being dissipated, the BL will reach an energy equilibrium and stop growing. Consequentially, more power would equal larger plasma balls. My record is a 10cm diameter sphere that lasted little over 3 minutes, which brings me to the phenomena of disappearing BL: It disappears when it reaches a "dead spot": a place where the microwaves are at their lowest energy. That happens because microwave ovens radiate irregularly, so some regions receive less energy than others. If you keep it from moving around, it won't disappear, but it will melt whatever confinement you have...